The present invention relates generally to printing, and more particularly to a method for detecting a shorted printhead in a printer having at least two printheads.
Printers include, without limitation, computer printers, copiers, and facsimile machines. Some printers, such as inkjet printers, print by printing closely-spaced ink dots on a print medium such as paper. Conventional inkjet printers include those having a carrier with two (or more) printheads such as a color printhead and a mono or a photo printhead. Typically, a color printhead prints cyan, magenta and yellow dots, a mono printhead prints black dots, and a photo printhead prints black, cyan and magenta dots. In one known design, the two (or more) printheads are coupled in parallel to the output of a voltage source such as the output of a printhead regulator (power adapter) to which a capacitance load has also been coupled in parallel. In normal operation, the regulator keeps the capacitor charged, and the printheads pull energy from the capacitor. A perfect printhead would have an infinite electrical resistance, and hence have no leakage current in a quiescent state. Actual printheads experience some leakage current in a quiescent state. A predetermined maximum leakage current is determined which indicates that the printhead is a shorted printhead and should be replaced.
Conventional methods for detecting a shorted printhead in a two printhead inkjet printer include detecting the current on the ground-return of the power adaptor and indicating a shorted printhead when the leakage current exceeds the predetermined maximum leakage current for a printhead. However, this method can indicate there is a shorted printhead in the two printhead inkjet printer when individual testing of each printhead would indicate each printhead is not a shorted printhead because the quiescent resistance of two printheads in parallel is less than the quiescent resistance of one printhead with the other printhead removed from the printer. This method can lead to confusion and cause a user to discard two good printheads or discard a good printhead and keep a shorted printhead.
What is needed is an improved method for detecting a shorted printhead in a printer having at least two printheads.
One method of the invention is for detecting at least one possibly shorted printhead in a printer having N printheads in parallel which are supplied a voltage from the output of a voltage source. This method includes steps a) through g). Step a) includes obtaining a calibration resistor having a resistance which, when placed in parallel to the voltage source, is equivalent to a predetermined maximum leakage current of a single non-shorted printhead in a quiescent state. Step b) includes placing the calibration resistor and a capacitance load in parallel across the output of the voltage source to define a first circuit. Step c) includes, with the N printheads electrically isolated from the first circuit, determining a first decay time for the first-circuit voltage across the capacitance load to reach a second voltage from a first voltage after the voltage source is disconnected from the first circuit. Step d) includes determining a second decay time which is shorter than the first decay time. Step e) includes placing the N printheads and the capacitance load in parallel across the output of the voltage source to define a second circuit. Step f) includes, with the calibration resistor electrically isolated from the second circuit and with the N printheads in a quiescent state, determining the second-circuit voltage across the capacitance load at the second decay time after the voltage source is disconnected from the second circuit. Step g) includes indicating at least one possibly shorted printhead of the N printheads when the second-circuit voltage at the second decay time is less than the second voltage. In one extension of this method, if step g) indicated at least one possibly shorted printhead, there are also included the steps of testing one printhead at a time with the other printheads removed from the printer and indicating that the one printhead is a shorted printhead if the voltage at the first decay time is less than the second voltage.
Another method of the invention is for detecting at least one possibly shorted printhead in a printer having N printheads in parallel which are supplied a voltage from the output of a voltage source. This method includes steps a) through g). Step a) includes obtaining a calibration resistor having a resistance which, when placed in parallel to the voltage source, is equivalent to a predetermined maximum leakage current of a single non-shorted printhead in a quiescent state. Step b) includes placing the calibration resistor and a capacitance load in parallel across the output of the voltage source to define a first circuit. Step c) includes, with the N printheads electrically isolated from the first circuit, determining a first decay time for the first-circuit voltage across the capacitance load to reach a second voltage from a first voltage after the voltage source is disconnected from the first circuit. Step d) includes determining a third voltage which is less than the second voltage. Step e) includes placing the N printheads and the capacitance load in parallel across the output of the voltage source to define a second circuit. Step f) includes, with the calibration resistor electrically isolated from the second circuit and with the N printheads in a quiescent state, determining the second-circuit voltage across the capacitance load at the first decay time after the voltage source is disconnected from the second circuit. Step g) includes indicating at least one possibly shorted printhead of the N printheads when the second-circuit voltage at the first decay time is less than the third voltage. In one extension of this method, if step g) indicated at least one possibly shorted printhead, there are also included the steps of testing one printhead at a time with the other printheads removed from the printer and indicating that the one printhead is a shorted printhead if the voltage at the first decay time is less than the second voltage.
Several benefits and advantages are derived from one or more of the methods of the invention. Using, with the same RC circuit decay voltage limit, a shorter decay time when testing two printheads than when testing one printhead or using, with the same RC circuit decay time, a lower decay voltage limit when testing two printheads than when testing one printhead allows the detection of at least one possibly shorted printhead with fewer false short indications than using conventional two printhead short detection methods.